The present invention relates to a superconducting magnet apparatus for, e.g., a synchrotron orbital radiation device.
For cooling a superconducting coil for a superconducting magnet apparatus, immersion cooling of immersing a superconducting coil in a coolant and cooling it with the latent heat of evaporation of the coolant, and direct cooling with a refrigerator are generally used.
FIG. 1 is an example of a superconducting magnet apparatus employing immersion cooling and shows a superconducting magnet apparatus for a synchrotron orbital radiation device. The superconducting magnet apparatus shown in FIG. 1 comprises a pair of superconducting coils 1. A radiation shield 2 surrounds the superconducting coils 1, and a high temperature-side shield 3 and a vacuum vessel 4 surround the radiation shield 2.
The superconducting coils 1 are respectively stored in coil containers 18, and a helium container 6 containing liquid helium 5 as a coolant and the coil containers 18 communicate with each other through pipes 6a. The superconducting coils 1 are immersed in the liquid helium 5 and held at a temperature of about 4.2 K. A helium liquefying refrigerator 7 is mounted on the helium container 6 to liquefy evaporated helium of the liquid helium 5 again.
The shield cooling refrigerator 8 cools the radiation shield 2 and high temperature-side shield 3 with a low temperature-side stage 8a and a high temperature-side stage 8b, respectively, and hold them at temperatures of 20 K and about 80 K, respectively. A beam chamber 9 is enclosed within a beam chamber radiation shield 10 and then by a beam chamber high temperature-side radiation shield 11.
During ordinary operation, the superconducting coils 1 have no electric resistance and do not generate heat. When there is influx of heat into the superconducting coils 1 from the outside by convection, conduction, or radiation, the heat that has entered the system is removed by evaporation of the liquid helium 5, and the evaporated helium is liquefied again by the helium liquefying refrigerator 7.
FIG. 2 shows an example of a superconducting magnet for direct cooling with a refrigerator. Referring to FIG. 2, a superconducting coil 1 is supported by heat insulating support members 26 and surrounded by a radiation shield 2. The radiation shield 2 is surrounded by a vacuum vessel 4. A low temperature-side stage 7a of a refrigerator 7 is thermally connected to the superconducting coil 1 through a heat conducting member 12, and a high temperature-side stage 7b thereof is thermally connected to the radiation shield 2. The low and high temperature-side stages 7a and 7b are respectively cooled to temperatures of about 4.2 K and 80 K. In this manner, since the refrigerator direct cooling type superconducting magnet apparatus does not use liquid helium 5, it is easy to handle and is suitable as a comparatively compact superconducting magnet apparatus. The refrigerator 7 for holding a temperature of 4.2 K currently has a capacity of as low as about 1 W and thus cannot be used for a large superconducting magnet apparatus.
In this superconducting magnet apparatus, the superconducting coil 1 is cooled to about 4.2 K by heat conduction with the low temperature-side stage 7a of the refrigerator 7 through the heat conducting member 12, so that its electric resistance becomes zero to reach a so-called superconducting state. In this state, an energizing current is supplied to the superconducting coil 1 from an external power supply (not shown) to generate a required magnetic field.
During ordinary operation, since the superconducting coil 1 has no electric resistance, the superconducting coil 1 does not generate heat by itself with Joule heat even if a current is supplied to it. However, there is influx of heat into the superconducting coil 1 from the outside by convection, conduction, or radiation. As described above, since the cooling capacity of one refrigerator 7 is limited, in the case of the refrigerator direct cooling type superconducting magnet apparatus, it is desired to decrease this heat invasion as much as possible.
In the conventional superconducting magnet apparatus that employs immersion cooling, as shown in FIG. 1, superconducting coils 1 are immersed in the liquid helium 5 to be cooled by its latent heat of evaporation. While this apparatus has high cooling characteristics, its liquid helium 5 is difficult to handle.
More specifically, prior to the operation, the liquid helium 5 must be reserved in the coil containers 18 that store the superconducting coils 1. This must be done by a person skilled in the art who has a necessary qualification. When the superconducting coils 1 are quenched (shift from superconduction to normal conduction) by a disturbance, they generate a very large Joule heat, and the reserved liquid helium 5 evaporates instantaneously. Generally, evaporated helium gas is stored in an external gas back temporarily or is discharged to the atmosphere. In this manner, when the superconducting coils 1 are quenched, liquid helium 5 must be supplied to the helium container 6 again.
The amount of liquid helium 5 to be used must be decreased as much as possible. However, in the case of immersion cooling, the use amount of liquid helium 5 is often determined by the size of the coil containers 18 depending on the size of the superconducting coils 1, and an optimum amount of helium liquid is not always stored. This causes a difficulty in handling and poses a problem in terms of conservation of natural resources as well.
Since the superconducting magnet apparatus employing direct cooling with a refrigerator as shown in FIG. 2 does not use liquid helium, it does not require liquid supplying operation and the like and can thus be handled easily. However, the cooling capacity of this apparatus is determined by the capacity of the mounted refrigerator 7. Generally, the superconducting coil 1 generates no heat while a constant current is supplied to it. However, during energization/deenergization such as turning ON/OFF, heat is generated by a large AC loss. When turning ON/OFF is very slow and takes a long period of time (from several ten minutes to 1 hour), cooling with the refrigerator can be performed. However, in a superconducting magnet apparatus that must be energized/deenergized within a short period of time (within several ten minutes), the AC loss sometimes reaches 10 times or more the heat influx.
Therefore, the number of refrigerators 7 must be increased, or a refrigerator 7 having a large capacity must be loaded to remove heat generated by AC loss. AC loss occurs only during short-time energization/deenergization, and such a measure is very uneconomical when considering long-term ordinary operation. When a large superconducting coil 1 is to be employed or a plurality of superconducting coils 1 are to be cooled with one refrigerator 7, as the refrigerator 7 and the superconducting coils 1 are thermally connected to each other through the heat conducting member 12, a temperature difference occurs among the respective portions of the superconducting coil 1 or among the respective superconducting coils 1 to cause quenching.